Liquid water transport in parallel serpentine channels with manifolds on cathode side of a PEM fuel cell stack

Water management in a proton exchange membrane (PEM) fuel cell stack has been a challenging issue on the road to commercialization. This paper presents a numerical investigation of air–water flow in parallel serpentine channels on cathode side of a PEM fuel cell stack by use of the commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different air–water flow behaviours inside the serpentine flow channels with inlet and outlet manifolds were discussed. The results showed that there were significant variations of water distribution and pressure drop in different cells at different times. The “collecting-and-separating effect” due to the serpentine shape of the gas flow channels, the pressure drop change due to the water distribution inside the inlet and outlet manifolds were observed. Several gas flow problems of this type of parallel serpentine channels were identified and useful suggestions were given through investigating the flow patterns inside the channels and manifolds.     

Innovative cathode flow-field design for passive air-cooled polymer electrolyte membrane (PEM) fuel cell stacks

A novel cathode flow-field design suitable for a passive air-cooled polymer electrolyte membrane (PEM) fuel cell stack is proposed to enhance the water-retaining capability under excess dry air supply conditions. The innovative cathode flow-field is designed to supply more air to the cooling channels and further enables deceleration of the reactant air in the gas channels and acceleration of the coolant air in the cooling channels simultaneously along the air flow path. Therefore, the design facilitates the waste heat removal through the cooling channels while the water removal by the reactant air is minimized. The conceptual cathode flow-field design is validated using a three-dimensional PEM fuel cell model. The detailed simulation results clearly demonstrate that the new cathode flow-field design exhibits superior water-retaining capability compared with a conventional cathode flow-field design (parallel flow channel configuration) under typical air-cooled fuel cell operating conditions. This study provides a new strategy to design cathode flow-fields to alleviate notorious membrane dehydration and unstable performance issues in a passive air-cooled PEM fuel cell stack.     

Gravity effect on the performance of PEM fuel cell stack with different gas manifold positions

The importance of gravity effect on the performance of proton exchange membrane fuel cell (PEMFC) has recently been recognized. In this paper, the effect of gravity on the performance of PEMFC has been investigated associating with different gas intake modes. The polarization curves of the stack with different positions of reaction gas inlet and outlet at varied gravitational angles are addressed in detail. The results indicate that the output power of PEMFC stack can be greatly enhanced at the optimized gravitational angle. Gas intake modes that were realized by varying the gas inlet and outlet positions strongly affect the stack performance as well. The optimized performance can be reached at the tilted angle of 90° when both air and hydrogen inlets are placed at the upper side of the stack, whereas the worst performance occurs at the tilted angle of 90° when air and hydrogen flow into the channel from the bottom side of the stack. These results have important implications for PEM fuel cell design and operational strategies. In order to improve the performance, fuel cells should be designed and operated at the optimized gravitational angle and gas inlet/outlet position. Copyright © 2011 John Wiley & Sons, Ltd.     

Numerical investigation of pressure drop characteristics of gas channels and U and Z type manifolds of a PEM fuel cell stack

Fluid flow manifold plays a significant role in the performance of a fuel cell stack because it affects the pressure drop, reactants distribution uniformity and flow losses, significantly. In this study, the flow distribution and the pressure drop in the gas channels including the inlet and outlet manifolds, with U- and Z-type arrangements, of a 10-cell PEM fuel cell stack are analyzed at anode and cathode sides and the effects of inlet reactant stoichiometry and manifold hydraulic diameter on the pressure drop are investigated. Furthermore, the effect of relative humidity of oxidants on the pressure drop of cathode are investigated. The results indicate that increase of the manifold hydraulic diameter leads to decrease of the pressure drop and a more uniform flow distribution at the cathode side when air is used as oxidant while utilization of humidified oxidant results in increase of pressure drop. It is demonstrated that for the inlet stoichiometry of 2 and U type manifold arrangement when the relative humidity increases from 25% to 75%, the pressure drop increases by 60.12% and 116.14% for oxygen and air, respectively. It is concluded that there is not a significant difference in pressure drop of U- and Z-type arrangements when oxygen is used as oxidant. When air is used as oxidant, the effect of manifold type arrangement is more significant than other cases, and increase of the stoichiometry ratio from 1.25 to 2.5 leads to increase of pressure drop by 527.3%.     

Numerical investigation of the impact of two-phase flow maldistribution on PEM fuel cell performance

Flow maldistribution usually happens in PEM fuel cells when using common inlet and exit headers to supply reactant gases to multiple channels. As a result, some channels are flooded with more water and have less air flow while other channels are filled with less water but have excessive air flow. To investigate the impact of two-phase flow maldistribution on PEM fuel cell performance, a Volume of Fluid (VOF) model coupled with a 1D MEA model was employed to simulate two parallel channels. The slug flow pattern is mainly observed in the flow channels under different flow maldistribution conditions, and it significantly increases the gas diffusion layer (GDL) surface water coverage over the whole range of simulated current densities, which directly leads to poor fuel cell performance. Therefore, it is recommended that liquid and gas flow maldistribution in parallel channels should be avoided if possible over the whole range of operation. Increasing the gas stoichiometric flow ratio is not an effective method to mitigate the gas flow maldistribution, but adding a gas inlet resistance to the flow channel is effective in mitigating maldistribution. With a carefully selected value of the flow resistance coefficient, both the fuel cell performance and the gas flow distribution can be significantly improved without causing too much extra pressure drop.     

Spatially resolved optical measurements of water partial pressure and temperature in a PEM fuel cell under dynamic operating conditions

In situ non-intrusive measurements of water vapor partial pressure and temperature were performed simultaneously along two gas channels on the cathode side of a PEM fuel cell using tunable diode laser absorption spectroscopy. This measurement technique developed by us was utilized earlier to make measurements in a single bipolar plate channel of a prototype PEM fuel cell. The current study examines the variation of water partial pressure and temperature near the flow inlet and outlet during operation under both steady state and time-varying load conditions. For steady-state operation, an increase in the water vapor partial pressure was observed with increasing current density due to electrochemical production of water. As expected, the measurement channel near the inlet of the flow path showed a lower water vapor partial pressure than the outlet under identical load conditions; however, the quantitative distribution of water content across the cell is important to understanding operational behavior of a PEM fuel cell. These quantitative water concentration differences between two measurement channels are reported with variations in cell load and temperature. Temperature in the gas phase remained constant due to thermal equilibrium of the fuel cell. For time varying operation, no phase lag was observed between the load and the water vapor partial pressure. The outlet measurement channel showed higher partial pressure than the inlet with larger differences for increasing cell load. The transient data matched the steady-state measurements at the same conditions. A temperature rise from the controlled value was observed at high current densities for the unsteady operation; thus, the temperature did not equilibrate on the same time scale as the water partial pressure.     

PEM fuel cell stacks operated under dry-reactant conditions

A double-path-type flow-field design that has two gas inlets and two gas outlets is presented for PEM fuel cells. The two paths are arranged in such a way that the inlet of one flow-field is adjacent to the outlet of the other flow-field and, within any section of the electrode active area, there are always adjacent channels with reactant flowing in opposite directions. Such a design enables the dry entering gas to become hydrated by acquiring some moisture from the exiting moist gas; and, within any section of the active area, the drier gas in one flow-field can share the moisture in the wetter gas flowing in the other flow-field. Such a design effectively uses the water produced by the stack to hydrate the membrane and the catalyst layers. The effectiveness of this design was demonstrated by running multiple-cell stacks where the stack could run stably at a current density up to 0.33 A/cm² using dry hydrogen and dry air.     

Design of an air humidifier for a 5 kW proton exchange membrane fuel cell stack operated at elevated temperatures

Air humidification is a crucial issue for superior performance of proton exchange membrane fuel cell (PEM fuel cell) stacks. In this work, an air humidifier is proposed for a 5 kW PEM fuel cell stack working at elevated temperatures, e.g., 90–95 °C. The high temperature coolant exiting the stack is utilized to pre-heat the air in the heat exchanging tubes of the humidifier, and the heated air is humidified with deionized water supplied by a nozzle fixed in a top cavity. Both the tubes and the nozzle are properly designed to ensure sufficient heat transfer and superior atomization. Humidification performance is evaluated under different operation conditions. The nozzle is able to inject well-atomized water with uniform droplet diameter. With the variation of inlet air flow rate, the relative humidity (RH) of the outlet air increases at the beginning, then decreases gradually due to the attenuation of dew point (DP) temperature. However, the humidification performance can be improved when higher temperature deionized water is injected or high temperature coolant is supplied. At a coolant temperature of 95 °C, the outlet air DP temperature is maintained over 80 °C with 25 °C injection water. Moreover, better humidification performance is achieved when the injection water flow rate is controlled according to the working conditions of the stack.     

Study of gas pressure and flow rate influences on a 500 W PEM fuel cell,thanks to the experimental design methodology

The behaviour of a 500 W PEM fuel cell stack, fed by pure hydrogen and humidified compressed air, is currently investigated on the fuel cell test platform of Belfort.In this paper, the influences on fuel cell performance of gas pressure and flow rate parameters are studied. The fuel cell is operated in the pressure regulation mode: the gas flow rates are regulated thanks to mass flow controllers placed upstream of the stack and the gas pressures at stack inlets are controlled by regulation valves located downstream of the stack. The choice of the various tests to perform is made thanks to experimental design methodology, which is a suitable technique to characterise, analyse and to improve a complex system such as a fuel cell generator. In this study, the four physical factors considered are both hydrogen/air pressures and anode/cathode flow rates. Each factor has two levels, leading to a full factorial design requiring 16 experiments (16 current–voltage curves). The test bench developed at the laboratory allows setting the other factors (for instance: stack temperature, relative humidity and dew point temperature of the air at stack inlet) at fixed values. The test responses are the maximal output power and the efficiency computed for this power. Statistical sensitivity analyses (ANOVA analyses) are used to compute the effects and the contributions of the various factors to the fuel cell maximal power. The use of fractional designs shows also how it is possible to reduce the number of experiments. Some graphic representations are employed in order to display the results of the statistical analyses made for different current values.     

Modelling of polymer electrolyte membrane fuel cell stacks based on a hydraulic network approach

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of hydrogen and oxygen directly into electrical energy. Waste heat and water are the reaction by‐products, making PEM fuel cells a promising zero‐emission power source for transportation and stationary co‐generation applications. In this study, a mathematical model of a PEM fuel cell stack is formulated. The distributions of the pressure and mass flow rate for the fuel and oxidant streams in the stack are determined with a hydraulic network analysis. Using these distributions as operating conditions, the performance of each cell in the stack is determined with a mathematical, single cell model that has been developed previously. The stack model has been applied to PEM fuel cell stacks with two common stack configurations: the U and Z stack design. The former is designed such that the reactant streams enter and exit the stack on the same end, while the latter has reactant streams entering and exiting on opposite ends. The stack analysed consists of 50 individual active cells with fully humidified H₂ or reformate as fuel and humidified O₂ or air as the oxidant. It is found that the average voltage of the cells in the stack is lower than the voltage of the cell operating individually, and this difference in the cell performance is significantly larger for reformate/air reactants when compared to the H₂/O₂ reactants. It is observed that the performance degradation for cells operating within a stack results from the unequal distribution of reactant mass flow among the cells in the stack. It is shown that strategies for performance improvement rely on obtaining a uniform reactant distribution within the stack, and include increasing stack manifold size, decreasing the number of gas flow channels per bipolar plate, and judicially varying the resistance to mass flow in the gas flow channels from cell to cell. Copyright © 2004 John Wiley & Sons, Ltd.     

Direct measurement of current density under land and two channels in PEM fuel cells with interdigitated flow fields

Interdigitated flow field is one of the commonly used designs in proton exchange membrane (PEM) fuel cells. The knowledge of how the current density differs under the inlet channel, the land and the outlet channel, is critical for flow field design and optimization. In this study, the current densities under the inlet channel, the land and the outlet channel in PEM fuel cell with an interdigitated flow field are separately measured using the technique of partially-catalyzed membrane electrode assemblies (MEAs). The experimental results show that the current density under the outlet channel is significantly lower than that under the inlet channel, and the current density under the land is higher than both channels at typical fuel cell operation voltages. Further experimental results show that the pattern of local current density remains the same with different cathode flow rates.     

An air-cooled proton exchange membrane fuel cell with combined oxidant and coolant flow

Combining the oxidant and coolant flow in an air-cooled proton exchange membrane fuel cell can significantly simplify the fuel cell design. In this paper, an air-cooled PEM fuel cell stack with an open cathode flow field, which supplied the oxidant and removed the heat produced in the fuel cell, was fabricated and tested. The influence of different operating parameters on cell voltage performance and the overall cell ohmic resistance, such as cell temperature and airflow rate, was investigated. The cell temperature and the temperature difference between the cell and the hydrogen humidifier were shown to serve important roles in reducing the fuel cell ohmic resistance. The test results also showed a noteworthy temperature gradient between each cell of a 5-cell stack. A hydrophilic treatment of the cathode flow field channels was demonstrated to be an effective way to mitigate water management issues caused at elevated operating temperatures.     

The experimental analysis of a dead-end H2/O2 PEM fuel cell stack with cascade type design

Proton exchange membrane fuel cells (PEMFCs) with a dead-ended anode and cathode can reach high hydrogen and oxygen utilization by a relatively simple system. Nevertheless, the accumulation of the water in the anode and cathode channels can lead to a local fuel starvation deteriorating the performance and the durability of PEMFCs. In this study, a novel design for a polymer electrolyte membrane (PEM) fuel-cell stack was presented which could achieve higher fuel utilization without using hydrogen and oxygen recirculation devices such as hydrogen pumps or ejectors that consume parasitic power and require additional control schemes. The basic concept of the innovatively proposed design was to divide the cells of a stack into several stages by conducting the outlet gas of each stage to a separator and reentering it into the next stage; thereby, a multistage anode and cathode system was prepared. In this relatively ingenious design, a higher gaseous flow rate was maintained at the cell outlet, even under dead-end conditions resulted in a reduced purge-gas emission by avoiding the accumulation of liquid water in the cells. The results revealed that proposed design had the same polarization curve as the open-end mode, leading to an enhanced PEMFC performance.     

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.     

Numerical modeling of manifold design and flow uniformity analysis of an external manifold solid oxide fuel cell stack

Computational fluid dynamics (CFD) technique and experimental measurement are combined to investigate the effects of several geometric parameters on flow uniformity and pressure distribution in an external manifold solid oxide fuel cell (SOFC) stack. The model of numerical simulation is composed of channels, tubes and manifolds based on a realistic 20-cell stack. Analysis results show that gas resistance in the channel can improve the flow uniformity. However, channel resistance only has a limited effect under high mass flow rate. With the increase of inlet tube diameter, the flow uniformity improves gradually but this has little impact on pressure drop. On contrary, the larger diameter of outlet tube reduces the pressure drop effectively with minor improvement on flow uniformity. The dimensions of the flared inlet tube and the round perforated sheet in the manifold are designed to optimize both flow uniformity and pressure drop. Practical experimental stack is established and the velocity in the outlet of the channel is measured. The trends of the experimental measurements are corresponding well with the numerical results. The investigation emphasizes the importance of geometric parameters to gas flow and provides optimized strategies for external manifold SOFC stack.     

A critical review of two-phase flow in gas flow channels of proton exchange membrane fuel cells

Water management in PEM fuel cells has received extensive attention due to its key role in fuel cell performance. The unavoidable water, from humidified gas streams and electrochemical reaction, leads to gas-liquid two-phase flow in the flow channels of fuel cells. The presence of two-phase flow increases the complexity in water management in PEM fuel cells, which remains a challenging hurdle in the commercialization of this technology. Unique water emergence from the gas diffusion layer, which is different from conventional gas-liquid two-phase flow where water is introduced from the inlet together with the gas, leads to different gas-liquid flow behaviors, including pressure drop, flow pattern, and liquid holdup along flow field channels. These parameters are critical in flow field design and fuel cell operation and therefore two-phase flow has received increasing attention in recent years. This review emphasizes gas-liquid two-phase flow in minichannels or microchannels related to PEM fuel cell applications. In situ and ex situ experimental setups have been utilized to visualize and quantify two-phase flow phenomena in terms of flow regime maps, flow maldistribution, and pressure drop measurements. Work should continue to make the results more relevant for operating PEM fuel cells. Numerical simulations have progressed greatly, but conditions relevant to the length scales and time scales experienced by an operating fuel cell have not been realized. Several mitigation strategies exist to deal with two-phase flow, but often at the expense of overall cell performance due to parasitic power losses. Thus, experimentation and simulation must continue to progress in order to develop a full understanding of two-phase flow phenomena so that meaningful mitigation strategies can be implemented.     

Improving dynamic performance of proton-exchange membrane fuel cell system using time delay control

Transient behaviour is a key parameter for the vehicular application of proton-exchange membrane (PEM) fuel cell. The goal of this presentation is to construct better control technology to increase the dynamic performance of a PEM fuel cell. The PEM fuel cell model comprises a compressor, an injection pump, a humidifier, a cooler, inlet and outlet manifolds, and a membrane-electrode assembly. The model includes the dynamic states of current, voltage, relative humidity, stoichiometry of air and hydrogen, cathode and anode pressures, cathode and anode mass flow rates, and power. Anode recirculation is also included with the injection pump, as well as anode purging, for preventing anode flooding. A steady-state, isothermal analytical fuel cell model is constructed to analyze the mass transfer and water transportation in the membrane. In order to prevent the starvation of air and flooding in a PEM fuel cell, time delay control is suggested to regulate the optimum stoichiometry of oxygen and hydrogen, even when there are dynamical fluctuations of the required PEM fuel cell power. To prove the dynamical performance improvement of the present method, feed-forward control and Linear Quadratic Gaussian (LQG) control with a state estimator are compared. Matlab/Simulink simulation is performed to validate the proposed methodology to increase the dynamic performance of a PEM fuel cell system.     

Effects of electrode wettabilities on liquid water behaviours in PEM fuel cell cathode

Liquid water transport is one of the key challenges for water management in a proton exchange membrane (PEM) fuel cell. Investigation of the air–water flow patterns inside fuel cell gas flow channels with gas diffusion layer (GDL) would provide valuable information that could be used in fuel cell design and optimization. This paper presents numerical investigations of air–water flow across an innovative GDL with catalyst layer and serpentine channel on PEM fuel cell cathode by use of a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different static contact angles (hydrophilic or hydrophobic) were applied to the electrode (GDL and catalyst layer). The results showed that different wettabilities of cathode electrode could affect liquid water flow patterns significantly, thus influencing on the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown, several gas flow problems were observed, and some useful suggestions were given through investigating the flow patterns.     

Experimental investigations on liquid water removal from the gas diffusion layer by reactant flow in a PEM fuel cell

The cross flow from channel to channel through gas diffusion layer (GDL) under the land could play an important role for water removal in proton exchange membrane (PEM) fuel cells. In this study, characteristics of liquid water removal from GDL have been investigated experimentally, through measuring unsteady pressure drop in a cell which has the GDL initially wet with liquid water. The thickness of GDL is carefully controlled by inserting various thicknesses of metal shims between the plates. It has been found that severe compression of GDL could result in excessive pressure drop from channel inlet to channel outlet. Removing liquid water from GDL by cross flow is difficult for GDL with high compression levels and for low inlet air flow rates. However, effective water removal can still be achieved at high compression levels of GDL if the inlet air flow rate is high. Based on different compressed GDL thicknesses, different GDL porosities and permeabilities were calculated and their effects on the characteristics of liquid water removal from GDL were evaluated. Visualization of liquid water transport has been conducted by using transparent flow channel, and liquid water removal from GDL under the land was observed for all the tested inlet air flow rates, which confirms that cross flow is practically effective to remove the liquid water accumulated in GDL under the land area.     

Application of Open Pore Cellular Foam for air breathing PEM fuel cell

Open Pore Cellular Foam (OPCF) has received increased attention for use in Proton Exchange Membrane (PEM) fuel cells as a flow plate due to some advantages offered by the material, including better gas flow, lower pressure drop and low electrical resistance.In the present study, a novel design for an air-breathing PEM (ABPEM) fuel cell, which allows air convection from the surrounding atmosphere, using OPCF as a flow distributor has been developed. The developed fuel cell has been compared with one that uses a normal serpentine flow plate, demonstrating better performance.A comparative analysis of the performance of an ABPEM and pressurised air PEM (PAPEM) fuel cell is conducted and poor water management behaviour was observed for the ABPEM design.Thereafter, a PTFE coating has been applied to the OPCF with contact angle and electrochemical polarisation tests conducted to assess the capability of the coating to enhance the hydrophobicity and corrosion protection of metallic OPCF in the PEM fuel cell environment. The results showed that the ABPEM fuel cell with PTFE coated OPCF had a better performance than that with uncoated OPCF.Finally, OPCF was employed to build an ABPEM fuel cell stack where the performance, advantages and limitations of this stack are discussed in this paper.